Ambient air temperature determination as a function of aircraft skin temperature.
Many aircraft control functions require ambient air temperature information. Some of these applications require relatively accurate data. For example, U.S. Pat. No. 4,303,978, entitled Integrated-Strapdown-Air-Data Sensor System, issued to Shaw, et al on Dec. 1, 1981, the complete disclosure of which is incorporated herein by reference, discloses the use of raw air temperature signals, produced by total air temperature sensors, which are compensated and manipulated to produce signals suitable for use by the automatic flight control, pilot display, and navigation systems of the aircraft. Another, more recent example shown in FIG. 1, U.S. Pat. No. 5,276,326, entitled Scanning Ranging Radiometer for Weather Hazard Surveillance, issued to Philpott on Jan. 4, 1994, the complete disclosure of which is incorporated herein by ireference, discloses an in-flight windshear detection system which utilizes an externally mounted array of photoconductive infrared detectors 102 to detect the air temperature 104 in front of the aircraft 106 so as to provide a more accurate indication of windshear during approach. Placing air temperature sensors outside the skin of an aircraft requires piercing the airframe with one or more mounting and signal access holes to lead the signal back to the onboard equipment operating the application, for example, a central processing unit 108. The need of such applications for highly accurate ambient air temperature information justifies the holes in the airframe and the consequent structure analysis effort.
Furthermore, the skin temperature of high speed commercial aircraft, unlike that of slower single-engine piston aircraft, is not directly related to the ambient air temperature. Therefore, those applications on commercial aircraft requiring only approximate air temperature data are forced to use one of the conventional airframe-piercing thermal probes and suffer the consequential expense. Examples include the density altimeter disclosed in U.S. Pat. No. 4,263,804, entitled Apparatus for Directly Measuring Density Altitude in an Aircraft , issued to Seemann on Apr. 28, 1981, the complete disclosure of which is incorporated herein by reference, which uses the ambient air temperature, pressure and humidity conditions to determine density altitude. In another example, U.S. Pat. No. 4,325,123, entitled Economy Performance Data Avionic System, issued to Graham, et al on Apr. 13, 1982, the complete disclosure of which is incorporated herein by reference, an avionic system is disclosed for producing the most economical engine and airspeed settings as a function of multiple parameters, including outside air temperature at the departure airport, which is essentially aircraft skin temperature, even in larger commercial aircraft. Still another example is the thermal navigation device disclosed in U.S. Pat. No. 4,591,111, entitled Thermal Navigator, issued to Laughter on May 27, 1986, the complete disclosure of which is incorporated herein by reference, which uses temperature sensors mounted on the right and left aircraft wings to provide a temperature differential between the wings and indicate a rate of change in ambient air temperature, whereby an ultralight aircraft, glider, or sailplane detects thermal updrafts and indicates to the pilot when and how rapidly to turn to obtain the maximum lift from the thermal updraft. Such applications, although useful on any general aviation aircraft, except perhaps U.S. Pat. No. 4,591,111, have not been generally practical on smaller aircraft because neither the expenses for equipment and installation of conventional airframe-piercing thermal probes nor the concomitant structural analysis can be justified.
However, practice of these and similar applications on ultralight aircraft, gliders, sailplanes, and slower single-engine piston aircraft depends upon the availability of a low-cost thermal sensor that need not pierce the airframe to provide an adequate measure ambient air temperature.
Some applications on aircraft also benefit by the use of an outside air temperature signal. For example, a geometric altitude computation in an Enhanced Ground Proximity Warning System (EGPWS) is more accurate when the pressure altitude component is enhanced using approximate air temperature data. The usefulness of merely approximate air temperature data in such an applications makes piercing of the airframe similarly impractical.
Thermistors are one well-known generally low-cost thermal sensor. A thermistor as defined by Webster""s New Collegiate Dictionary, 1977 edition, published by G.andC. Merriam Company, Springfield, Mass., is an electrical resistor making use of a semiconductor whose resistance varies sharply in a known manner with temperature. Conventional thermistor chips are shown in FIGS. 2A, 2B and described, for example, in U.S. Pat. No. 5,952,911, entitled Thermistor Chips and Methods of Making Same, issued to Kawase, et al on Sep. 14, 1999, the complete disclosure of which is incorporated herein by reference, as chips 110 formed of terminal electrodes 112 provided at both end parts of a thermistor element 114 having an oxide of a transition metal such as Mn, Co and Ni as its principal component. The terminal electrodes 112 are each formed of an end electrode 112A formed by applying Ag/Pd or the like in a paste form and then firing and a plating layer 112B formed on its surface by using Ni or Sn. The normal-temperature resistance value of such a thermistor chip is generally determined by the resistor value of the thermistor element 114 and the position of the terminal electrodes 112. As shown in FIG. 2C, U.S. Pat. No. 5,952,911 also discloses a more recent thermistor chip 116 wherein the terminal electrodes 118 are formed of a multi-layer structure having an inner most layer 118A in direct contact with a surface of the thermistor element 114 that affects its normal temperature resistance value.
However, thermistors have not been in use in aircraft industry for measuring ambient air temperature. Nor have either conventional thermal sensors or thermistors been used to measure skin temperature to determine ambient air temperature, without piercing the airframe. Therefore, a need exists to provide low precision ambient air temperature information in a form which avoids piercing the airframe skin and the concomitant structural analysis.
The present invention overcomes the limitations in the prior art of determining ambient air temperature using conventional thermal probes that pierce the airframe by providing a method for generating a signal representative of ambient air temperature surrounding an aircraft and supplying that signal to an avionics application by attaching a thermal sensor in contact with an interior surface of the skin of the aircraft, and with said thermal sensor, generating a signal as a function of ambient air temperature. According to one aspect of the invention, the thermal sensor is generally chosen from the class of thermal sensors known as thermistors.
According to other aspects of the invention, the thermistor is fixed in intimate thermal contact with the interior surface of the aircraft skin. Such intimate thermal contact is provided by installing a thermally conductive material between the thermistor and the interior skin surface. Preferably, the thermally conductive material is a thermally conductive bonding agent. The thermistor supplies the generated signal to one or more avionics applications capable of proper functioning using an imprecise indication of ambient air temperature.
According to still other aspects of the invention, the invention includes protecting the thermistor from an interior atmosphere of aircraft to reduce thermal effects resulting from contact with the usually warmer interior atmosphere. For example, the invention includes covering portions of the thermistor otherwise exposed to the interior atmosphere of the aircraft.
According to other aspects of the invention, the signal generated by the thermistor is directly correlated to ambient air temperature. However, the invention includes the option of conditioning the signal. According to one aspect of the invention, the signal is conditioned as a function of aircraft altitude, such as an applied offset that varies with altitude. According to an alternative aspect of the invention, the signal is conditioned using a constant offset.
According to still other aspects of the invention, the invention provides a low-precision ambient air temperature measuring device for use with avionics applications capable of functioning with imprecise air temperature data, the device embodying the method of the invention and including both a thermal sensor fixed in contact with an inner surface of the skin of an aircraft and a means for detecting a temperature signal generated by the thermal sensor.
According to one aspect of the invention, the device of the invention includes a layer of material thermally coupling the thermal sensor to the inner skin surface. Preferably, the thermal coupling material is a layer of thermally conductive material, for example, a thermally conductive bonding agent fixing the thermal sensor in intimate thermal contact with the inner skin surface.
According to another aspect of the invention, the thermal sensor of the device is a thermal sensor generally chosen from the class of thermal sensors known as thermistors.
According to yet another aspect of the invention, the invention includes a protective cover installed over the thermal sensor. Preferably the protective cover includes a surface, such as a rim, lip or flange, that contacts the inner skin surface in an area surrounding and adjacent to the thermal sensor""s position, so that the cover and skin surface together essentially encompass and enclose the thermal sensor. The cover thereby protects the thermal sensor from damage, while protecting it also from effects of contact with the interior atmosphere of the aircraft. According to one aspect of the invention, the cover is at least in part formed of a thermally insulating material, such as ceramic or plastic.